1. Field of the Invention
This invention relates to gas mixing valves, and more particularly to mixing valves adapted for use in a medical respirator.
2. Description of the Prior Art
The task of mixing air and oxygen in desired ratios for medical respirators is complicated by the fact that the air and oxygen supplies are usually subject to pressure fluctuations, and do not remain consistently in balance. This is not a significant problem at high flow rates when the pressure drop across the mixing valve is considerably larger than the expected range of imbalance between the two gas sources. However, as the volumetric gas flow rate decreases and the pressure drop across the valve correspondingly decreases, the effect of air and oxygen supply pressure imbalance on the mixing accuracy is greatly increased. While a high capacity mixing valve may have a relative mixing accuracy comparable to the relative mixing accuracy of smaller capacity valves, i.e., each valve is characterized by comparable percentage errors in the mixing accuracies, the larger capacity valves are inherently less accurate on an absolute scale. In other words, the mixing error in liters per minute as opposed to a percentage of the total flow is worse for higher capacity than for lower capacity valves. This becomes a problem should an attempt be made to operate a larger valve at a lower flow rate, since the absolute error caused by fluctuations in the gas supply pressures will then become significant in proportion to the total flow. Although mixing accuracy can be enhanced by the use of a relatively small scale valve, this unduly limits the range of flow rates over which the valve is capable of operating.
An approach toward solving this problem is described in U.S. patent application Ser. No. 677,344, assigned to the assignee of the present invention. In this application the dimensions of the orifices through which air and oxygen flow into the valve are variable in a first direction to adjust the mixing ratio. The orifice dimensions are also variable in a second direction in response to a pressure sensitive device to adjust the overall area through which the gases flow into the valve. The reduction in accuracy during low flow rates is thereby substantially overcome by decreasing the size of both inlet orifices, which in turn increases the pressure drop across the orifices. This pressure sensing function is performed by a diaphragm which adjusts the dimensions of the air and oxygen orifices by equal proportionate amounts.
While the above type of mixing valve represents a significant improvement, its components require careful machining in order to achieve the necessary close tolerances. There is still a need, however, for a lower cost gas mixing valve which combines an ability to operate over a wide range of flow rates with an improved absolute mixing accuracy at the lower flow rates.